The invention concerns a method of creating pores of nanometric to micrometric size in a thin sheet of polyimide, as well as a thin sheet of polyimide obtained by this method.
Methods of creating pores of nanometric to micrometric size in a polymeric material in sheet form, such as ethylene polyterephthalate or polycarbonate, are already known. However, these materials do not exhibit great thermal resistance. Polycarbonate, for example, has much less good properties beyond 150° C.
Conversely, polyimides preserve their excellent physical, electrical and mechanical properties in a very wide range of temperatures, and in particular at very high (beyond 400° C.) or very low temperatures. Thus the use of polyimides is particularly sought in certain applications, and new applications may even be envisaged. By way of example, thin sheets of polyimide comprising pores of nanometric to micrometric size can be used as filtering membranes, as a support for synthesizing filaments or tubes, in particular nanofilaments or nanotubes, in the connection field, etc.
The known methods of creating pores of nanometric to micrometric size in a polymeric material in sheet form are for example described in the international patent applications published under the numbers WO 01/49402 and WO 01/49403, from the applicant. These methods comprise the successive steps consisting of subjecting the sheet to:                an ion bombardment;        a UV radiation;        chemical etching.However, these methods do not successfully apply to polyimides. This is because the applicant has found that the use of this method on sheets of polyimide results in the obtaining of pores of very different diameters and non-cylindrical shape, which is not desirable in many applications. The shape and the consistency of the diameter of the pores can be improved, but chemical etching of several hours is then necessary.        
The aim of the invention is to resolve these problems by providing a method for creating pores with a substantially cylindrical shape and substantially equal diameters in a thin sheet of polyimide, the method not requiring a step of chemical etching of long duration. To this end, and according to a first aspect, the invention concerns a method of creating pores of nanometric to micrometric size in a thin sheet of polyimide, in which the sheet is subjected, in this order, to:                an ion bombardment;        a radiation in the visible range;        a chemical etching.        
The irradiation of the sheet in the visible range increases the quantity of damage caused in the sheet along the path of the ions previously bombarded on the said sheet—which is not the case with UV irradiation, as found by the applicant. As a result the chemical etching of the sheet is more rapid at this damage, which leads to the formation of pores of substantially cylindrical shape and substantially equal diameters.
According to the invention, the sheet is subjected to a radiation whose wavelength is between 360 and 550 nm, or even between 450 and 530 nm. According to one possible embodiment, use is made of a source emitting in the visible range and which gives off a sufficiently small quantity of heat not to require cooling of the sheet. For example, the sheet is subjected to the radiation of one or more LEDs, emitting at wavelengths between 450 and 530 nm, for a period of between 1 and 10 hours. The sheet can also be subjected to the radiation of a halogen spot lamp for a period of around from 10 hours to one week, and to the radiation of a UV-Hg source coupled to a UV filter, emitting at wavelengths of between 360 and 580 nm.
According to other characteristics, the sheet is subjected to chemical etching for less than one hour, or even less than 15 minutes. The chemical etching can be carried out by a solution of NaOCl with a concentration of between 0.01 N and 5 N at a temperature of between 30° and 100° C. After having been subjected to the ion bombardment and before being subjected to the radiation in the visible range, the sheet can be subjected to global heat treatment, certain predetermined areas of the sheet only being subjected to the said radiation, so as to effect a “patterning”.
Finally, according to a second aspect, the invention concerns a thin sheet of polyimide comprising pores of nanometric to micrometric size, obtained by the method as previously described, the pores being substantially cylindrical and having substantially equal diameters. The sheet has for example a thickness of between 2 and 100 μm. According to another embodiment, the sheet has a thickness of between 5 nm and 10 μm, and is deposited on a support.